There are several types of field-driven particles used in the field of non-emissive displays. One class uses the so-called electrophoretic particle that is based on the principle of movement of charged particles in an electric field. In an electrophoretic receiver, the charged particles containing different reflective optical densities can be moved by an electric field to or away from the viewing side of the receiver, which produces a contrast in the optical density. Another class of field-driven particles are particles carrying an electric dipole. Each pole of the particle is associated with a different optical densities (bi-chromatic). The electric dipole can be aligned by a pair of electrodes in two directions, which orient each of the two polar surfaces to the viewing direction. The different optical densities on the two halves of the particles thus produces a contrast in the optical densities.
To produce a high quality image on a receiver having field-driven particles, it is desirable to produce multiple or continuous tone optical densities at each pixel. Tone scale is particularly important for displaying pictorial images.